First of all, thank you very much for your kind words and for taking the time to write, I really do appreciate it!

Hmm, you mention the ability to gain access to your backup. This, in itself, does not require that you boot from the hard drive in question: simply connecting it and transferring files should be enough to migrate your data — or let the transfer assistant do it.

Is there any specific reason for which you wanted to boot from that external drive?